1. Field of the Invention
[0001] This invention relates generally to an extreme ultraviolet (EUV) lithography source
and, more particularly, to an EUV source that employs an input laser beam positioned
off-axis or asymmetrically relative to the first collection optics to improve the
fraction of produced EUV radiation.
Discussion of the Related Art
[0002] Microelectronic integrated circuits are typically patterned on a substrate by a photolithography
process that is well known to those skilled in the art, where the circuit elements
are defined by a light beam propagating through a mask. As the state of the art of
the photolithography process and integrated circuit architecture becomes more developed,
the circuit elements become smaller and more closely spaced together. As the circuit
elements become smaller, it is necessary to employ photolithography light sources
that generate light beams having shorter wavelengths and higher frequencies. In other
words, the resolution of the photolithography process increases as the wavelength
of the light source decreases to allow smaller integrated circuit elements to be defined.
The current trend for photolithography light sources is to develop a system that generates
light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13.4nm).
[0003] Different devices are known in the art to generate EUV radiation. One of the most
popular EUV radiation sources is a laser-plasma, gas condensation source that uses
a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton,
and combinations of gases, are also known for the laser target material. The gas is
forced through a nozzle, and as the gas expands, it condenses and converts to a liquid
spray. The liquid spray is illuminated by a high-power laser beam, typically from
an Nd:YAG laser, that heats the liquid droplets to produce a high temperature plasma
which radiates the EUV radiation. U.S. Patent No. 5,577,092 issued to KUBIAK discloses
an EUV radiation source of this type.
[0004] Figure 1 is a plan view of a known EUV radiation source 10 including a nozzle 12
and a laser beam source 14. A gas 16 flows through a neck portion 18 of the nozzle
12 from a gas source (not shown). The gas is accelerated through a narrowed throat
portion and is expelled through an exit collimator of the nozzle 12 as a jet spray
26 of liquid droplets. A laser beam 30 from the source 14 is focused by focusing optics
32 on the liquid droplets. The heat from the laser beam 30 generates a plasma 34 that
radiates EUV radiation 36. The nozzle 12 is designed so that it will stand up to the
heat and rigors of the plasma generation process. The EUV radiation 36 is collected
by collection optics 38 and is directed to the circuit (not shown) being patterned.
The collection optics 38 can have any suitable shape for the purposes of collecting
and directing the radiation 36. In this design, the laser beam 30 propagates through
an opening 40 in the collection optics 38.
[0005] It has been shown to be difficult to produce a spray having large enough droplets
of liquid to achieve the desired efficiency of conversion of the laser radiation to
the EUV radiation. Because the liquid droplets have too small a diameter, and thus
not enough mass, the laser beam 30 causes some of the droplets to break-up before
they are heated to a sufficient enough temperature to generate the EUV radiation 36.
Typical diameters of droplets generated by a gas condensation EUV source is on the
order of 0.33 microns. However, droplet sizes of about 1 micron in diameter would
be desirable for generating the EUV radiation. Additionally, the large degree of expansion
required to maximize the condensation process produces a diffuse jet of liquid, and
is inconsistent with the optical requirement of a small plasma size.
[0006] To overcome the problem of having sufficiently large enough liquid droplets as the
plasma target, U.S. Patent application Serial No. (Attorney Docket No. 11-1119), filed
August 23, 2000, titled "Liquid Sprays as the Target for a Laser-Plasma Extreme Ultraviolet
Light Source," discloses a laser-plasma, extreme ultraviolet light source for a photolithography
system that employs a liquid spray as a target material for generating the laser plasma.
In this design, the EUV source forces a liquid, preferably Xenon, through the nozzle,
instead of forcing a gas through the nozzle. The geometry of the nozzle and the pressure
of the liquid propagating through the nozzle atomizes the liquid to form a dense spray
of liquid droplets. Because the droplets are formed from a liquid, they are larger
in size, and are more conducive to generating the EUV radiation.
[0007] Sources for EUV lithography based on laser produced plasma currently employ laser
beams that are symmetric with the axis of the first collection optics. Hardware, including
the nozzle, diffuser, etc., that provides the target material for the laser beam is
positioned proximate the focal point of the first collection optics because the plasma
generation area must be located at this position. The nozzle is positioned orthogonal
to the laser beam. In this position, the hardware obscures the EUV radiation reflected
from the central portion of the optics. This is because the EUV radiation generated
from the plasma has an angular distribution that is strongly peaked in the direction
of the incoming laser beam and decreases to nearly zero at angles orthogonal to the
laser beam. Hence, the region of the strongest EUV illumination at the collection
optics cannot reflect to subsequent optics, resulting in a substantial decrease in
the fraction of EUV radiation that can be utilized.
[0008] Figure 2 is a schematic plan view of a known EUV radiation source 50 from a different
angle than the source 10 shown in Figure 1 that demonstrates this problem. In this
example, a nozzle and associated target production hardware 52 is shown positioned
relative to a plasma spot 54. The target laser beam 56 propagates through an opening
58 in collection optics 60, where the axis of the laser beam is symmetric relative
to the shape of the optics 60. The collection optic 60 is generally dish-shaped having
a reflective surface shape suitable for the purposes described herein. In this configuration,
the angular distribution 62 of the produced EUV radiation causes the strongest EUV
radiation to propagate towards the collection optics 60 in a direction directly opposite
to propagation direction of the laser beam 56. Thus, the stronger EUV radiation reflected
from the optics 60 is directed back towards the target production hardware 52 and
the weaker EUV radiation is reflected at the edges of the collection optics 60. Thus,
the target production hardware 52 blocks much of the strong EUV radiation, which results
in a significant loss of this radiation.
[0009] What is needed is a design change of the known EUV source that does not obscure a
significant portion of the generated EUV radiation so as to increase the fraction
of EUV radiation that is usable. It is therefore an object of the present invention
to provide such a source.
SUMMARY OF THE INVENTION
[0010] In accordance with the teachings of the present invention, an EUV source is disclosed
that delivers the laser beam to the plasma generation area off-axis relative to the
first collection optics. Particularly, the first collection optics has an opening
for the laser beam at a location so that laser beam is directed towards the plasma
generation area at an angle that is off-axis relative to the collection optics. Thus,
the strongest EUV radiation is not blocked by the target production hardware. In one
embodiment, the collection optics is a section of a dish, where the direction of the
laser beam causes the strongest EUV radiation to be reflected from the outer edges
of the optics. In another embodiment, the collection optics is a full dish having
two openings for two separate laser beams to generate EUV radiation sent in a direction
so it is also reflected at the outer edges of the optics.
[0011] Additional objects, advantages and features of the present invention will become
apparent to those skilled in the art from the following discussion and the accompanying
drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Figure 1 is a plan view of a known laser-plasma, gas condensation extreme ultraviolet
light source;
[0013] Figure 2 is a schematic plan view of a known EUV source where the input laser beam
is symmetric relative to the first collection optics;
[0014] Figure 3 is a schematic plan view of an EUV source where a single input laser beam
is provided off-axis relative to the first collection optics, according to an embodiment
of the present invention; and
[0015] Figure 4 is a schematic plan view of an EUV source where two input laser beams are
provided off-axis relative to the first collection optics, according to another embodiment
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] The following discussion of the preferred embodiments directed to an EUV lithography
source where an input laser beam is delivered off-axis relative to the first collection
optics is merely exemplary in nature, and is in no way intended to limit the invention
or its applications or uses.
[0017] Figure 3 is a plan view of an EUV source 66 shown at an angle similar to that of
the source 50 shown in Figure 2, according to an embodiment of the present invention.
In -this embodiment, target production hardware 68 is positioned at its usual location
relative to a plasma spot 70. However, the collection optics 60 had been replaced
with first collection optics 72 that is only partially dish-shaped and is positioned
at a different location relative to the hardware 68 than the collection optics 60.
In this embodiment, the collection optics 72 is about half the size of the collection
optics 60, and is positioned above the target hardware 68. An opening 74 is provided
in the collection optics 72 through which a target laser beam 76 propagates to the
plasma spot 70. The input laser beam 76 is positioned off-axis or asymmetrical relative
to the collection optics 72. Because the collection optics 72 is at this position,
the angular distribution 78 of the generated EUV radiation directed towards the collection
optics 72 is such that the strong EUV radiation is reflected from the upper edges
of the optics 72 and is not obscured by the target hardware 68, as shown.
[0018] In the embodiment shown in Figure 3, the collection optics 72 is about one-half the
size of the collection optics 60 shown in Figure 2. Therefore, some of the generated
EUV radiation does not get reflected from the collection optics 72 that normally would
in the conventional system. Figure 4 is a schematic plan view of an EUV source 82
including target hardware 84 and a plasma spot 86, according to another embodiment
of the present invention. In this embodiment, first collection optics 88 has the same
shape as the collection optics 60, but includes two openings 90 and 92 for two separate
input laser beams 94 and 96, respectively. Basically, it is the embodiment shown in
Figure 3, only doubled so that strong EUV radiation is provided both above and below
the target hardware 84. Particularly, the angular distribution 98 of the EUV radiation
from the beam 94 is directed along the line of the input laser beam 94 and is reflected
from the optics 88 below the target hardware 84, and the angular distribution 100
of the EUV radiation from the beam 96 is directed along the line of the input laser
beam 96 and is reflected above the target hardware 84. Therefore, the embodiment shown
in Figure 4 provides more EUV radiation than the EUV source 50.
[0019] The foregoing discussion describes merely exemplary embodiments of the present invention.
One skilled in the art would readily recognize that various changes, modifications
and variations can be made therein without departing from the spirit and scope of
the invention as defined in the following claims.
1. A laser-plasma extreme ultraviolet (EUV) radiation source comprising:
a nozzle emitting a spray of target material into a plasma generation region; and
collection optics positioned relative to the plasma generation region, said collection
optics including at least one opening through which a laser beam propagates to impinge
the target material and generate a plasma, said opening being positioned at a location
asymmetrical relative to the collection optics so that the laser beam is directed
off-axis relative to the collection optics and most of the strongest EUV radiation
from the plasma reflected by the collection optics is not obscured by the nozzle.
2. The source according to claim 1 wherein the collection optics is dish-shaped.
3. The source according to claim 1 wherein the collection optics is a portion of a dish
shape.
4. A laser-plasma extreme ultraviolet (EUV) radiation source comprising:
a nozzle emitting a spray of target material into a plasma generation region; and
collection optics positioned relative to the nozzle, said collection optics including
at least one opening through which a laser beam propagates to impinge the target material
and generate a plasma, said opening being positioned at a location asymmetrical relative
to the collection optics so that the laser beam is directed off-axis relative to the
collection optics and most of the strongest EUV radiation from the plasma reflected
by the collection optics is not obscured by the nozzle but is disposed such that the
angular distribution of the generated EUV radiation causes most of the strongest EUV
radiation to be directed towards an edge of the collection optics.
5. A laser-plasma extreme ultraviolet (EUV) radiation source comprising:
a nozzle emitting a spray of target material into a plasma generation region; and
collection optics positioned relative to the plasma generation region, said collection
optics including a single opening through which a single laser beam propagates to
impinge the target material and generate a plasma, said opening being positioned at
a location asymmetrical relative to the collection optics so that the laser beam is
directed off-axis relative to the collection optics and most of the strongest EUV
radiation from the plasma reflected by the collection optics is not obscured by the
nozzle.
6. The source according to claim 1 wherein the collection optics includes two separate
openings, each receiving a separate laser beam.
7. A laser-plasma extreme ultraviolet (EUV) radiation source for generating EUV radiation,
said source comprising:
target production hardware including a nozzle, said nozzle emitting a spray of target
material into a plasma generation region;
a laser beam source, said laser beam source generating a laser beam directed towards
the plasma generation region; and
collection optics positioned between the laser beam source and the target production
hardware, said collection optics including at least one opening through which the
laser beam propagates to impinge the target material and generate a plasma, said collection
optics having a shape that is at least a portion of a dish, said plasma generation
region being at a focal point of the collection optics and said target production
hardware being proximate the focal point of the collection optics, said opening being
positioned at a location asymmetrical relative to the shape of the collection optics
so that the laser beam is directed off-axis relative to the collection optics and
most of the strongest EUV radiation from the plasma reflected by the collection optics
is not obscured by the target production hardware.
8. A laser-plasma extreme ultraviolet (EUV) radiation source for generating EUV radiation,
said source comprising:
target production hardware including a nozzle, said nozzle emitting a spray of target
material into a plasma generation region;
a laser beam source, said laser beam source generating a laser beam directed towards
the plasma generation region; and
collection optics positioned between the laser beam source and the target production
hardware, said collection optics including at least one opening through which the
laser beam propagates to impinge the target material and generate a plasma, said collection
optics having a shape that is at least a portion of a dish, said plasma generation
region being at a focal point of the collection optics and said target production
hardware being proximate the focal point of the collection optics, said opening being
positioned at a location asymmetrical relative to the shape of the collection optics
so that the laser beam is directed off-axis relative to the collection optics and
most of the strongest EUV radiation from the plasma reflected by the collection optics
is not obscured by the target production hardware and wherein the collection optics
is positioned relative to the nozzle such that the angular distribution of the generated
EUV radiation causes most of the strongest EUV radiation to be directed towards an
edge of the collection optics.
9. The source according to claim 8 wherein the collection optics is dish-shaped.
10. The source according to claim 8 wherein the collection optics includes two separate
openings, each receiving a separate laser beam.
11. A method of generating extreme ultraviolet (EUV) radiation, said method comprising
the steps of:
providing target production hardware including a nozzle;
emitting a spray of target material from the nozzle into a plasma generation region;
directing at least one laser beam to the plasma generation region to heat the target
material and generate the EUV radiation; and
reflecting the generated EUV radiation from collection optics, said step of directing
the laser beam including directing the laser beam through an opening in the collection
optics so that the laser beam is off-axis relative to the collection optics and most
of the strongest EUV radiation is not obscured by the target production hardware.
12. A method of generating extreme ultraviolet (EUV) radiation, said method comprising
the steps of:
providing target production hardware including a nozzle;
emitting a spray of target material from the nozzle into a plasma generation region;
directing at least one laser beam to the plasma generation region to heat the target
material and generate the EUV radiation; and
reflecting the generated EUV radiation from collection optics, said step of directing
the laser beam including directing the laser beam through an opening in the collection
optics so that the laser beam is off-axis relative to the collection optics and most
of the strongest EUV radiation is not obscured by the target production hardware and
is reflected from an edge of the collection optics.
13. The method according to claim 11 wherein the step of directing the laser beam includes
directing two separate laser beams through two separate holes in the collection optics,
where both laser beams are off-axis relative to the collection optics.